Comparative review of low-frequency acoustic properties of Western Mediterranean and Gulf of Mexico seagrass species

Field and laboratory experiments since 1995 have been directed at the development of a model for sound propagation through seagrass meadows. Seagrass blades are a complex structure of tissue and internal gas channels, which can produce oxygen bubbles in the water column during photosynthesis. Simple mixture theory models have proven ineffective at predicting the acoustic behaviour observed during field experiments. Since 2013, laboratory investigations have been directed at determining the tissue acoustic properties of main Mediterranean species Posidonia oceanica and Cymodocea nodosa using ultrasound and then low-frequency sound. This review focuses on the determination of the low-frequency (1-5 kHz) effective sound speed of a mixture of seagrass leaf blades and degassed artificial seawater placed in an acoustic resonator. All measurements from MEDGRASS 15 and 16 experiments are compared to previous ones of three Texas Gulf coast seagrass species as a function of added wet biomass. The ensemble of results shows correlation of this speed with species, and average size, physical stiffness, and epiphyte coverage of the leaf blades.